Ooh wait, if that's how we're doing it, then a baseline 2 hours, plus 1 hour for each spell level above 3rd still nets you a maximum of 8 hours, but the caster will be at a high-enough character level that other, more expedient options will usually have presented themselves by then.

My point exactly. You can overcast Magic Missile up to 9th-level too, if you really wanted. It'd do 12d4+12 damage, but I'm pretty there are better damage-dealing options at that spell level. From what I've seen, overcast spells scale linearly, whereas your spell options at higher levels are scaled non-linearly.